Gas-impermeable elastic membrane and hydropnematic accumulator equipped with this measure

ABSTRACT

A gas-impermeable elastic membrane including a co-polymeric gas-impermeable film including OH groups, positioned between two layers of a co-polymer containing blocks of polyamides and of polyesters, with a rubber coating on at least one of the two layers, as well as a method of manufacturing the membrane.

FIELD OF THE INVENTION

The present invention relates to an impermeable elastic membrane, to aprocess for its manufacture and to a hydropneumatic accumulator equippedwith this membrane and used, for example, in the suspensions of motorvehicles. Such an accumulator is in the form of a sphere separated by amembrane into two compartments, one of which contains a gas such asnitrogen and the other contains a liquid such as mineral oil. Under theeffect of the oil pressure, the membrane becomes deformed and compressesthe gas.

The main characteristics of the membrane are its flexibility,elasticity, impermeability to nitrogen, leaktightness and resistance tomineral hydraulic fluid.

BACKGROUND OF THE INVENTION

Two types of polyurethane-based membrane are currently used, for examplea thermoplastic polyurethane such as Desmopan® from Bayer or apolyurethane combined with rubber such as Urepan®. The durability ofthese membranes is satisfactory, but they have a drawback as regards thelow impermeability to nitrogen of polyurethane. This low impermeabilitylimits the useful life of accumulators of limited. Thus, after threeyears of use on a vehicle, nearly half of the rating pressure is lost.

It is sought to improve the impermeability to nitrogen of membranes.EP-A-360,648 discloses, in particular, a membrane having twice theimpermeability of the polyurethane-based one. This membrane contains twomaterials in combination, the first material giving the membrane therequired elasticity and being selected from thermoplastic polyurethanes,polyether block amides and flexible polyesters or a mixture of these,the second material being immersed in the bulk of the first material inorder to achieve the impermeability to gases and selected from anethylene/vinyl alcohol copolymer, polyamides and polyvinylidene chlorideor a mixture of these.

When an ethylene/vinyl alcohol copolymer is used as the second material,it is mixed with a polyether block amide, and the first material is athermoplastic polyurethane. The thickness of the film is, in this case,typically between 10 and 200 micrometers.

Novel hydropneumatic accumulators have appeared recently on the market.They are described in document FR-A-2,662,638. They contain twocompartments separated by a cold-resistant multilayer membrane forming agas barrier. The film forming the gas barrier is of the polyvinylalcohol type placed between two sheets of rubber. The role of the rubberis to ensure the leaktightness of the mounting of the membrane in theaccumulator. The polyvinyl alcohol film contains a polyol-typeplasticizer, for example glycerol, in a percentage of from 15 to 50% byweight.

These novel membranes are satisfactory, but owing to the fact that theplasticizer can migrate or evaporate, especially in the case of use athigh temperatures, it is necessary to insert a skin of vapor-impermeableand plasticizer-impermeable resin, for example a polyethylene skin,between the film of polyvinyl alcohol and the rubber. Furthermore, thesemembranes require specific storage and handling conditions, owing to thepresence of plasticizer.

FR-A-2,685,740 describes flexible membranes for hydropneumaticaccumulators, which consist of a layer (1) of ethylene/vinyl alcohol(EVOH) copolymer placed between two layers of polyamide (2), thetriple-layer itself being coated on each face with a layer of NBR rubber(3). In order to soften the layers of polyamide (2), it is necessary toadd EPDM thereto. The layers (2) do not adhere well to the layer (1). Itis also necessary to insert an adhesive between the layers (2) and (3).

SUMMARY OF THE INVENTION

It is an object of the invention to propose a hydropneumatic accumulatormembrane which makes it possible to avoid the drawbacks mentioned aboveand which is of high stability towards ageing at high temperature.

The subject of the invention is thus a gas-impermeable elastic membraneintended in particular to equip a hydropneumatic accumulator, andcomprising at least:

a film formed

of a skin of a gas-impermeable copolymer containing —OH groups,

and of two layers between which is incorporated the skin and whichconsist of a flexible and elastic material of a copolymer containingpolyamide blocks and polyether blocks, a rubber coating optionally beingplaced on at least one layer of the flexible and elastic material.

The membranes according to the invention are impermeable andthermoformable, have good heat resistance, low rigidification under coldconditions, and are flexible over a wide temperature range without itbeing necessary to add plasticizers to the gas-impermeable skin, or tothe layers of the copolymer containing polyamide blocks and polyetherblocks. Furthermore, depending on the types of rubber copolymer used, noadhesive or binder is necessary.

According to another characteristic of the invention, thegas-impermeable copolymer is a copolymer of a monomer of formulaCH₂═CH—R in which R is H or an alkyl group, with a monomer of formulaCH₂═CH—(R′)_(x)OH in which x is equal to 0 or 1 and R′ is an alkylgroup, and more particularly a copolymer of ethylene and of vinylalcohol.

According to an advantageous form, the membrane of the inventioncomprises a rubber coating on at least one of its faces and preferablyon both faces.

According to other characteristics of the invention:

the rubber is, for example, a nitrile rubber and contains carboxylic,amino or epoxy functional groups,

the melting point of the film formed by the skin and the layers offlexible and elastic material is greater than 160° C.,

the thickness of the skin is between 25 and 100 micrometers, and

the thickness of each layer of the flexible material and of the rubberis between 200 and 600 micrometers.

The invention also relates to a process for manufacturing such amembrane. The process consists in carrying out a coextrusion of the skinand the layers in order to obtain a film,. optional drying in order toremove water from the layers, optionally followed by thermoforming inorder to give the film the desired shape, and molding with vulcanizationof the rubber.

A coextrusion binder can be placed between the skin and the layers offlexible and elastic material.

The drying is preferably carried out for 24 hours at 70° C.

The molding with the rubber is carried out at a temperature below themelting point of the film formed by the skin and the layers.

The binding between the rubber and the film can be achieved using anadhesive which is placed beforehand on the outer faces of the film. Ifthe rubber has functional groups, it is not necessary to use anadhesive.

The adhesive comprises a non-polar solvent such as xylene or a ketone.

BRIEF DESCRIPTION OF THE DRAWING

The characteristics and advantages of the invention will become apparentfrom the following description, which is given purely by way of exampleand with reference to the attached single FIG. 1, is a cross-sectionalview of the membrane according to the preferred form of the invention.

The membrane represented in FIG. 1 comprises a skin 1 of a rigidmaterial which is impermeable to gases, more particularly to nitrogen.The skin 1 is inserted between two layers 2 and 3 consisting of aflexible and elastic material.

The gas-impermeable material is a partially crystalline copolymercontaining —OH groups. Advantageously, it comprises units

in which R is H or an alkyl group, and units

in which x is equal to 0 or 1 and R′ is an alkyl group.

EVOH is advantageously used.

EVOH is a copolymer consisting essentially of ethylene and vinyl alcoholunits-and can contain small amounts of other monomer units, inparticular vinyl ester units. These copolymers can be obtained by totalor partial saponification or alcoholysis of ethylene/vinyl estercopolymer. Among the vinyl esters, vinyl acetate is the preferredmonomer. The degree of saponification or of alcoholysis is at leastequal to 90 mol % and is preferably between 54 and 99.5 mol %. The molarproportion of ethylene in the EVOH is within the range from 3 to 75%,preferably from 10 to 50%.

It would not be considered a departure from the scope of the inventionto use mixtures of EVOH and of polyolefin containing an EVOH matrix.Such mixtures are described in the documents EP-A-418,129 andEP-A-441,666, the content of which is incorporated in the presentapplication.

It would also not be considered a departure from the scope of thepresent invention to replace all or part of the EVOH by EVOH modified byacylation or urethanization. The acylation is carried out with an acidchloride, an acid or an anhydride, advantageously with an acid chloridesuch as benzoyl chloride. The urethanization is advantageously carriedout with an isocyanate such as, for example, phenyl isocyanate or benzylisocyanate. Such products are described in application WO 95/12624, thecontent of which is incorporated in the present application.

EVOH has excellent impermeability to gases. For example, if it iscompared with other compounds and its permeability to O₂ at 20° C. ismeasured (in cc. 20 μ/m², 24 h, atm), the following results areobtained:

Compound Permeability Ethylene/vinyl alcohol (EVOH) copolymer 0.4 to 1.5Polyvinyl alcohol (PVAL) 0.1 Polyvinyl alcohol plasticized with 10glyceryl (35 parts) Polyamide (PA) 76 Polyester (PET) 69

EVOH has flexibility properties and is easy to use on account of thepresence of ethylene comonomer, when compared with other materials suchas PVAL which require a plasticizer for their conversion. This isbecause plasticizer-free PVAL degrades before melting. In contrast, EVOHcan be used free of plasticizer.

The skin 1 is incorporated between two layers 2, 3 consisting of aflexible and elastic material. A material which is flexible without theuse of a plasticizer and which has excellent hear and cold resistance ischosen. Such a material is preferably a copolymer containing polyamideblocks and polyether blocks, which will be referred to as PEBA(polyether block amide) in the text hereinbelow.

The role of this material is to provide thickness without providing toomuch rigidity to the film formed by the skin 1 and the layers 2, 3.Furthermore, the thermoplastic elastomer properties of the PEBA areused.

The polymers containing polyamide blocks and polyether blocks resultfrom the copolycondensation of polyamide blocks containing reactive endswith polyether blocks containing reactive ends, such as, inter alia:

polyamide blocks containing diamine chain ends with polyoxyalkyleneblocks containing dicarboxylic chain ends;

polyamide blocks containing dicarboxylic chain ends with polyoxyalkyleneblocks containing diamine chain ends, obtained by cyanoethylation andhydrogenation of aliphatic, α,Ω-dihydroxylated polyoxyalkylene blocksknown as polyetherdiols;

polyamide blocks containing dicarboxylic chain ends with polyetherdiols,the products obtained being, in this particular case,polyetheresteramides.

The blocks containing dicarboxylic chain ends are derived, for example,from the condensation of α,Ω-aminocarboxylic acids of lactams or ofdicarboxylic acids and diamines in the presence of a chain-limitingdicarboxylic acid. Advantageously, the polyamide blocks are polyamide-12(PA-12) or polyamide-6 (PA-6).

The number-average molar mass M_(n) of the polyamide blocks is between300 and 15,000 and preferably between 600 and 5000. The mass M_(n) ofthe polyether blocks is between 100 and 6000 and preferably between 200and 3000.

The polymers containing polyamide blocks and polyether blocks can alsocomprise randomly distributed units. These polymers can be prepared bysimultaneous reaction of the polyether and the precursors of thepolyamide blocks.

For example, a polyetherdiol, a lactam (or an α,Ω-amino acid) and achain-limiting diacid can be reacted together in the presence of a smallamount of water. A polymer is obtained essentially having polyetherblocks, polyamide blocks of very variable length, but also the variousreagents which have reacted randomly and are distributed randomly alongthe polymer chain.

Whether they are derived from the copolycondensation of polyamide blocksand polyether blocks prepared beforehand or from a one-step reaction,these polymers containing polyamide blocks and polyether blocks have,for example, Shore D hardnesses which can be between 20 and 75 andadvantageously between 30 and 70, and an intrinsic viscosity between 0.8and 2.5, measured in meta-cresol at 250° C. for an initial concentrationof 0.8 g/100 ml.

Whether the polyether blocks are derived from polyethylene glycol, frompolyoxypropylene glycol or from polyoxytetramethylene glycol, they areeither used as they are and copolycondensed with polyamide blockscontaining carboxylic ends, or they are aminated in order to beconverted into polyetherdiamines and condensed with polyamide blockscontaining carboxylic ends. They can thus be mixed with polyamideprecursors and a chain-limiting agent in order to make the polymerscontaining polyamide blocks and polyether blocks having randomlydistributed units.

Polymers containing polyamide-blocks and polyether blocks are describedin U.S. Pat. Nos. 4,331,786, 4,115,475, 4,195,015, 4,839,441, 4,864,014,4,230,838 and 4,332,920.

The polyether can be, for example, a polyethylene glycol (PEG), apolypropylene glycol (PPG) or a polytetramethylene glycol (PTMG). Thelatter is also known as polytetrahydrofuran (PTHF).

Whether the polyether blocks are in the chain of the polymer containingpolyamide blocks and polyether blocks in the form of diols or diamines,they are referred to for simplicity as PEG blocks or PPG blocks or elsePTMG blocks.

It would not be considered a departure from the scope of the inventionif the polyether blocks contained different units such as units derivedfrom ethylene glycol, from propylene glycol or from tetramethyleneglycol.

Preferably, the polymer containing polyamide blocks or polyether blockscomprises only one type of polyamide block and only one type ofpolyether block. Polymers containing PA-12 blocks and PTMG blocks andpolymers containing PA-6 blocks and PTMG blocks are advantageously used.

A mixture of two or more polymers containing polyamide blocks andpolyether blocks can also be used.

Advantageously, the polymer containing polyamide blocks and polyetherblocks is such that the polyamide is the major constituent by weight,i.e. the amount of polyamide which is in the form of blocks and thatwhich is possibly randomly distributed in the chain represents 50% byweight or more of the polymer containing polyamide blocks and polyetherblocks. Advantageously, the amount of polyamide and the amount ofpolyether are in the ratio (polyamide/polyether) from 1/1 to 3/1.

A coextrusion binder can optionally be placed between the skin ofimpermeable copolymer and the layers of flexible and elastic material.

By way of a example of a binder, mention may be made of:

polyethylene, polypropylene, copolymers of ethylene and of at least oneα-olefin, and mixtures of these polymers, all of these polymers beinggrafted with unsaturated carboxylic acid anhydrides such as, forexample, maleic anhydride. Mixtures of these grafted polymers and ofthese non-grafted polymers can also be used.

copolymers of ethylene with at least one product chosen from (i)unsaturated carboxylic acids, their salts and their esters, (ii) vinylesters of saturated carboxylic acids, (iii) unsaturated dicarboxylicacids, their salts, their esters, their hemiesters and their anhydrides,(iv) unsaturated epoxides; these copolymers can be grafted withunsaturated dicarboxylic acid anhydrides such as maleic anhydride orunsaturated epoxides such as glycidyl methacrylate.

The thickness of this binder can be from 15 to 30 μm. If the PEBAcomprises PA-6 blocks and PTMG blocks, it is not necessary to use abinder between the layers of PEBA and the skin of impermeable copolymer.

Due to the fact that EVOH contains no plasticizer, the film formed bythe skin 1 and the layers 2, 3 has excellent stability over time andheat stability. For example, if the stability of the film PEBA/EVOH/PEBAis compared with a film of polyvinyl alcohol plasticized with glycerol,the following results are obtained, after exposure to air at 100° C. for16 hours:

Variation of the Variation of the 50 modulus elongation PEBA/EVOH/PEBA +3%  +8% film PVAL film plasticized +300% −50% with glycerol

It will thus be evident, seen that the film according to the inventionis much more stable than the plasticized PVAL film.

Preferably, the thickness of the skin 1 is chosen so as to be as thin aspossible, in order for the film not to be too rigid, but so as to besufficient to give the assembly impermeability to gases. For example,the thickness of the skin 1 can be between 50 and 200 micrometers.

If a plasticized PVAL film having a thickness of 1300 micrometers iscompared with a film according to the invention consisting of a skin ofEVOH of 100 micrometers in thickness and of two layers of PEBA each of600 micrometers, for comparable modulus values, the level of gaspermeability is of the same order of magnitude:

Plasticized PVAL PEBA/EVOH/PEBA Thickness (in μm) 1300 600/100/600 25%modulus (in MPa)  14 12 Permeability (in 10⁻¹⁸ 5 × 10⁻¹⁸ m²/Pa · s)

Preferably, the thickness of each of the layers 2, 3 is between 200 and600 micrometers.

According to a particular form of the invention, a coating 4 is placedon at least one layer 2 of flexible and elastic material, but preferablyeach layer 2, 3 contains a coating 4, 5. This coating is made of rubber.The role of the rubber is to provide the resistance to the hydraulicfluid, namely the mineral oil, placed in a compartment of thehydropneumatic accumulator, and to provide the leaktightness of themembrane mounting in this accumulator.

By way of example of rubber, mention may be made of natural rubber,polyisoprene having a high level of double bonding in the cis position,a polymerized emulsion based on styrene/butadiene copolymer, apolybutadiene having a high level of double bonding in the cis position,obtained by nickel, cobalt, titanium or neodymium catalysis, ahalogenated ethylene/propylene/diene terpolymer, a halogenated butylrubber, a styrene/butadiene block copolymer, a styrene/isoprene blockcopolymer, the halogen-containing products of the above polymers, anacrylonitrile/butadiene copolymer, an acrylic elastomer, afluoroelastomer and chloroprene. Epichlorohydrin rubbers are also used.

Functionalized rubbers such as those having arboxylic acid, carboxylicacid anhydride, epoxide or mine groups are advantageously used. Carboxylor anhydride groups are preferred.

When the elastomers mentioned above contain no carboxylic acid radicalsor anhydride radicals of the said acids (which is the case for most ofthem), the radicals will be provided by grafting, in a known manner, ofthe elastomers mentioned above, or by mixtures of elastomers, forexample with elastomers containing grafted or copolymerized acrylicacid.

Among the elastomers mentioned above, those included in the followinggroup will advantageously be chosen: carboxylated nitrile elastomers,carboxylated acrylic elastomers, carboxylated polybutadienes, graftedethylene/propylene/diene terpolymers or mixtures of these polymers withthe identical but non-grafted elastomers, such as nitrile rubbers,polybutadienes and ethylene/propylene/diene terpolymers, alone or as amixture.

Preferably, the abovementioned vulcanizable elastomers contain a weightproportion of carboxylic acid radicals or dicarboxylic acid anhydrideradicals of between 0.3 and 100% relative to the elastomers.

The vulcanization systems which are suitable for the present inventionare well known to those skilled in the art and, consequently, theinvention is not limited to a specific type of system.

The rubbers can be modified by the addition of fillers such as carbonblack, silica, kaolin, aluminum oxide, clay, talc, chalk, etc. Thesefillers an be surface-treated with silanes, polyethylene glycols or anyother coupling molecule. In general, the proportion of fillers in partsby weight is between 5 and 100 per 100 parts of elastomers.

In addition, the compositions can be made flexible by plasticizers suchas mineral oils derived from petroleum, phthalic acid esters or sebacicacid esters, liquid polymer plasticizers such as polybutadiene of lowmass which is optionally carboxylated, and other plasticizers which arewell known to those skilled in the art.

The rubber is, for example, a nitrile rubber of formula:

which preferably contains carboxyl groups. Such a carboxylated nitrilerubber has the following formula (which is denoted by X-NBR):

By way of example, the rubber is a copolymer of 1,3-butadiene, ofacrylonitrile and of acrylic acid.

When the coatings 4, 5 are made of non-functionalized rubber, the layers2, 3 are attached together solidly by means of an adhesive 6, 7 placedon the outer faces of the layers 2, 3. Such an adhesive is chosen so asnot to act on the physical and/or chemical properties of the film formedby the skin 1 and the layers 2, 3. Thus, the adhesive will not containalcohol-type polar solvents such as ethanol, and an adhesive containinga non-polar solvent such as xylene or a ketone will be used.

The invention also relates to a process for manufacturing a membrane asdescribed above.

Direct coextrusion of a copolymer of ethylene and of vinyl alcohol witha polyether block amide is carried out in order to obtain a filmcontaining three layers, with a total thickness of between 0.425 and 1.3mm. The film is such that the central skin 1 has a thickness of between25 and 100 micrometers and is coated on each side with a flexiblematerial 2, 3 with a thickness of between 200 and 600 micrometers. Thisfilm contains no plasticizer and can be stored and packaged more easilyand shows better aging under hot conditions.

A coextrusion binder can optionally be placed between the skin and eachlayer of polyether block amide.

It is recommended to dry the film in order to remove the traces of watercontained in the PEBA and to improve the cohesion between the skin 1 andthe layers 2, 3. The drying is carried out at a temperature of 70° C.for 24 hours. The melting point of the film is greater than 160° C. inorder to ensure its subsequent use and its behavior during itssubsequent use.

If necessary, the film is thermoformed to give it the desired shape, inorder for it to fit into the hydropneumatic accumulator for which it isintended.

An adhesive is optionally placed on the outer faces of the film, afterwhich molding and vulcanization of the rubber are carried out on thefilm. Molding with the rubber is carried out at a temperature below themelting point of the film, for example below 160° C.

A carboxylated nitrile rubber can be used and, in this case, no adhesiveis placed on the outer faces of the membrane. The non-vulcanized rubberis molded on the film, i.e. on at least one of the layers of theflexible and elastic material optionally coated with an adhesive. Therubber which is molded on is more specifically a mixture containing thebase elastomer (for example the X-NBR), vulcanization agents andfillers. The vulcanization is carried out at a temperature below themelting point of the film. The assembly of the film coated with therubber is thus vulcanized at a temperature which is sufficient to bringabout vulcanization but below the melting point of the film.

The melting point of the skin and that of the layers of the flexiblematerial is generally from about 140 to 170° C. The duration of thevulcanization can be between a few minutes and 15 minutes.

Advantageously, the kinetics measured using an oscillating rheometerwill be such that the characteristic time for 90% vulcanization, t₉₀,does not exceed 15 minutes and will advantageously be between 5 and 10minutes.

Moreover, it has been found that the vulcanization start time (orsetting time) corresponding to an increase in torque of 0.2 N.m was animportant factor for obtaining materials showing high-qualityperformance. Thus, it is advantageous for the abovementioned increase intorque to be achieved in a time longer than or equal to 4 minutes at themolding temperature and preferably between 4 and 5 minutes.

The Applicant has observed that if the vulcanization was too rapid theadhesion to the film was poor.

As regards the film in which the layers of the flexible and elasticmaterial is PEBA, the vulcanization is advantageously carried out at atemperature close to the VICAT point of the PEBA.

The VICAT point, or the softening point, is a well-known parameter formeasuring the physical properties of a polymer. The VICAT point is thetemperature at which a needle with a circular cross-section of 1 mm²penetrates 1 mm into the sample during an increase in temperature of 50°C. per hour, according to ASTM standard D1525. Thus, at thistemperature, the polymer does not flow and is not in the molten state.

The molding followed by vulcanization is described in EP-A-550,346 andEP-A-607,085, the content of which is incorporated in the presentapplication.

As regards PEBAs in which the blocks are PA-6 and PTMG, the Applicanthas found that the vulcanization can be carried out over a much widertemperature range around the VICAT point; it is thus even easier to movefurther away from the melting point of the PEBA. This technique isdescribed in EP-A-682,060, the content of which is incorporated in thepresent application.

What is claimed is:
 1. A gas-impermeable elastic membrane, said membranecomprising at least: (a) a co-polymeric gas-impermeable film includingOH groups; and (b) two layers of a supple and elastic material of aco-polymer containing blocks of polyamides and blocks of polyethers,said film being positioned between said layers.
 2. The elasticimpermeable membrane set forth in claim 1, wherein the film includes aco-polymer of ethylene and vinyl alcohol.
 3. The elastic impermeablemembrane set forth in claim 1, wherein the thickness of the film isbetween 25 and 100 μm.
 4. The elastic impermeable membrane set forth inclaim 1, wherein the thickness of the supple and elastic layers isbetween 200 and 600 μm.
 5. The elastic impermeable membrane set forth inclaim 1, further including a rubber coating positioned on at least oneof said layers of elastic and supple material.
 6. The elasticimpermeable membrane set forth in claim 5, wherein the rubber coating islocated on each of the layers.
 7. The elastic impermeable membrane setforth in claim 5, wherein the rubber of the coating is a co-polymer of1.3 butadiene, of acrylonitrile and acrylic acid.
 8. A method formanufacturing a membrane comprising the steps of: (a) preparing a formedfilm having two layers by coextrusion; (b) thermal forming said film;(c) molding said film with non-vulcanized rubber as a coating; and (d)vulcanizing the rubber at a temperature sufficient to completevulcanization but below a fusion temperature.